Welding by radiofrequency energy is an efficient and fast way to manufacture certain products. For example, radiofrequency energy may be used to weld certain polymeric materials, such as polyvinyl chloride (PVC), to make flexible bags for retaining fluid. For example, a bag or bladder for receiving pressurized air is incorporated into a vascular compression device for preventing pulmonary embolisms and deep vein thrombosis (DVT).
A bladder of a typical vascular compression device includes a pair of opposing polymeric sheets welded around their perimeters and a polymeric tube port welded between the sheets in fluid communication with the bladder. An exemplary process for forming the bladder uses a bladder die for welding the bladder together and a cylindrical mandrel for welding the polymeric sheets to the tube. At a first weld station, the cylindrical mandrel is inserted into the tube, and the mandrel and the tube are placed between the opposing sheets. A tube die is lowered to compress the tube and the mandrel between the sheets. Radiofrequency current is supplied to the mandrel to create a radiofrequency electric field between the mandrel and the die. The electric field heats the polymeric sheets and the tube, thereby welding the sheets to the tube. After the sheets are welded to the tube, the mandrel is removed from the tube and the subassembly is moved to a second welding station for forming the perimeter of the bladder. The subassembly is compressed between two opposing die members and radiofrequency current is directed to the die members to form a perimeter-weld.
The use of a cylindrical mandrel is inefficient and time-consuming because of the difficulties in both inserting the mandrel into the tube and removing the mandrel from the tube after the process. Moreover, at least two distinct welding operations are required to form the bladder. Moreover still, the mandrel must be connected to a source of radiofrequency energy.
In another process, the mandrel is replaced by a rigid, non-deformable tubular insert that is received in the tube. Like the above process, radiofrequency energy is supplied to a die to create an electric field. However, in this process, the die includes portions that surround the tube and the tubular insert and direct the radiofrequency electric field into the tube and the sheets surrounding the tube to weld them together.
Although this process purportedly welds both the bladder and the tube to the bladder at the same time and in one step, the use of a tubular insert, without more, is not sufficient to weld both the bladder and the sheets to the tube. Welding the sheets to the tubing takes longer than welding the bladder because the tube is typically thicker than the polymeric sheet. If the process lasted long enough to adequately weld the sheets to the tubing, then there is a likelihood that the die will cut or at least weaken the bladder at the bladder perimeter because of the amount of time the sheets would be subjected to the electric field.